Sulfur is a natural constituent of many raw materials used in industry. Materials such as petroleum, coal, and metal ores contain natural sulfur compounds that may be inorganic or organic. During processing, the sulfur compounds are frequently, oxidized to sulfur dioxide and become part of a waste gas stream. Due to the noxious and poisonous nature of sulfur dioxide, it is necessary to limit the amount of sulfur dioxide that is disposed into the atmosphere as part of a waste gas stream and frequently it is desirable to remove the sulfur dioxide from such gas streams simply for the sake of recovering a valuable material.
In the case of high sulfur oxides content in a gas, catalytic sulfur dioxide conversion into sulfur trioxide is used for sulfuric acid production. The reaction is strongly dependent on temperature with regard to both rate and completeness of the reaction. The rate is basically slow and the catalyst must be used to increase it to commercially acceptable levels.
The most widely used catalyst is vanadium pentoxide. This is incorporated in an essentially inert base material which is formed into uniformly sized pellets or extrusions. Even with this catalyst, the rate of reaction is extremely slow at temperatures below 417.degree. C., which is called the ignition temperature for the catalyst. Once the reaction stars, or is ignited, the heat of reaction raises the gas temperature and the rate of reaction increases rapidly.
There is a limit to the allowable temperature rise because the catalyst can be permanently damaged at temperatures above 627.degree. C. Even below this temperature there is a slow loss of vanadium from the catalyst which eventually requires the replacement of the catalyst operating at the highest temperatures. Complete oxidation of sulfur dioxide to sulfur trioxide can never be achieved under practical conditions because there is always some reaction in the opposite direction. A 98% sulfur dioxide conversion into sulfur trioxide is only possible at the lower operating temperatures around 427.degree. C. At a temperature of 527.degree. C. the conversion is limited to about 90%, and at 627.degree. C. the limit is about 70%.
Removing sulfur dioxide from stack gases is a particularly difficult problem because .the stack gases are so large in volume compared to the amount of sulfur dioxide present, the gases are hot, and because the stack gases may contain ingredients that interfere with the sulfur dioxide removal process. Sulfur dioxide is an acid gas, but the usual processes for removing acid gases from a gas stream cannot be employed with stack gases. For example, the most prevalent and successful of such processes is to scrub the gas stream with an organic solvent; however hot stack gases decompose and vaporize organic materials so that the organic solvents cannot be used, or at least cannot be used economically in cleaning stack gases. In order to employ an organic liquid, it is necessary to cool the entire gas stream to a temperature that can be tolerated by the organic liquid, and after sulfur dioxide is removed, it is necessary to re-heat the entire gas stream to restore its buoyancy so it may rise up a stack and carry into the atmosphere. In some cases, blowing is used instead of re-heating, but an essential mount of energy is wasted. Both cooling and heating of a large volume of gas are difficult processes that cannot be justified economically for the removal of the small volume of sulfur dioxide contained in such a gas.
No single method of removing SO.sub.2 from large quantities of flue gas has a clear cut advantage over the others. The most used industry methods of SO.sub.2 removal from gases are listed below with basic reactions of interest:
a) limestone (and lime) scrubbing processes ##STR1## b) limestone scrubbing modified with magnesium sulfate ##STR2## c) magnesium oxide scrubbing EQU SO.sub.2 +H.sub.2 O+MgSO.sub.3 .fwdarw.Mg(HSO.sub.3).sub.2 EQU Mg(HSO.sub.3).sub.2 (heat).fwdarw.MgSO.sub.3 +SO.sub.2 +H.sub.2 O PA1 d) alkali scrubbing EQU Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.fwdarw.2NaHSO.sub.3 EQU NaHSO.sub.3 +0.50.sub.2 .fwdarw.Na.sub.2 SO.sub.4 PA1 e) citric acid scrubbing PA1 f) dry scrubbing
Water solution scrubbing methods from a) to e) produce wet stocks to be purified and throwaway sulfates (either calcium or alkali metals). In order to employ water solutions, it is necessary to cool the entire gas stream to a temperature below the water boiling temperature 100.degree. C.; and after sulfur dioxide is removed, it is necessary to re-heat the entire gas stream to restore its buoyancy so it may rise up a stack and carry into the atmosphere. Dry scrubbing processes don't produce wet stocks and exclude cooling--reheating stages, but they are very difficult to run and said processes produce calcium sulfate. Instead of alkali metals, aqueous ammonium hydroxide can be used in alkali scrubbing.
Ammonia injection is a known way to reduce NO.sub.x emissions according to the following reactions: EQU 4NH.sub.3 +6NO.fwdarw.5N.sub.2 +6H.sub.2 O EQU 8NH.sub.3 +6NO.sub.2 .fwdarw.7N.sub.2 +12H.sub.2 O
Temperature control must be maintained in the range of 200.degree.-320.degree. C. Below 200.degree. C. ammonium nitrate is formed. A major problem is ammonium hydrogen sulfate NH.sub.4 HSO.sub.4 (ammonium bisulfate) formation in presence of the sulfur oxides which leads to deposition on the pipe walls which cause a pressure drop and system clogging.
Chemical reactions imployed in the proposed invention are reactions between sulfate salts, sulfur oxides, and water to form hydrogen sulfates. Similar reactions of sulfite salts with sulfur dioxide and water to form hydrogen sulfites are well known and in practice in Ca and Mg scrubbing processes.
The purpose of the proposed invention is a combination of energetic advantages of molten salts systems (no gas cooling necessary for desulfurization followed by heating to achieve buoyancy) with simple effective chemical reactions. Moreover, the process consumes no inert gas and an ash is the only disposal waste produced, sulfuric acid is produced as a by-product, and simultaneous SO.sub.x and NO.sub.x removal may be achieved.